Biogas installation for production of biogas from biomass, and methods for operation of the biogas installation

ABSTRACT

A biogas installation for production of biogas from biomass and methods for starting and shutting down a fermenter are disclosed. When biomass in a fermenter is used up, biogas production must stop, the fermented biomass removed, and the fermenter filled with fresh biomass. Biogas production and utilization is maintained for as long as possible. When the methane concentration in the biogas outlet falls below an upper limit, the biogas line is disconnected. The biogas/off-gas mixture is fed out via an exhaust chimney until the methane concentration has fallen to a lower limit. The fermenter to be shut down is purged with fresh air, and the off-gas/biogas/fresh air mixture is fed out via the exhaust chimney until the carbon-dioxide concentration in the off-gas/biogas/fresh air mixture has fallen to a first limit. The fermenter is then opened to unload the consumed biomass and load fresh biomass.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 12/126,708,filed on May 23, 2008, the entire disclosure of which is incorporatedherein by reference.

FIELD OF THE INVENTION

The invention relates to a biogas installation for production of biogasfrom biomass having at least one fermenter, to a method for shuttingdown a fermenter, and to a method for starting a fermenter

BACKGROUND OF THE INVENTION

So-called “dry fermentation” allows pourable biomasses from agriculture,from biological waste and from communal cultivated areas to be convertedto methane without having to convert the materials to a liquid substratewhich can be pumped. Biomasses with a dry substance component of up to50% can be fermented. This dry fermentation method is described, forexample, in EP 0 934 998.

In the case of “dry” fermentation, the material to be fermented is notstirred into a liquid phase as is the case, for example, with liquidfermentation of bio waste. Instead of this, the fermentation substratewhich has been introduced into the fermenter is kept moist all the timeby taking the percolate at the bottom of the fermenter away and sprayingit over the biomass again. This results in optimum living conditions forthe bacteria. During the recirculation of the percolate, the temperaturecan also be regulated, and it is possible to add additives for processoptimisation.

WO 02/06439 discloses a bioreactor or a fermenter in the form of aprefabricated garage which is operated using the principle of dryfermentation in the so-called batch process. In this case, after seedingwith already fermented material, the fermenter is filled with thefermentation substrate by means of tractor shovels. The fermentationcontainer is constructed in the form of a garage and is closed by agastight door. The biomass is fermented with air being excluded, with nofurther thorough mixing during the process, and with no additionalmaterial being supplied. The percolate which seeps out of the materialbeing fermented is drawn off via a drainage groove, is temporarilystored in a tank, and is sprayed over the fermentation substrate again,in order to moisturise it. The fermentation process takes place in themesophilic temperature range between 34 and 37° C., with the temperaturebeing created by means of bottom heating and wall heating.

The resultant biogas can be used to obtain electricity and heat in acogeneration system that generates heat and electric power at the sametime. In order to ensure that sufficient biogas is always available forthe cogeneration system, a plurality of fermentation containers areoperated with offset timings in the dry fermentation installation. Atthe end of the dwell time, the fermenter area is completely emptied andis then refilled. The fermented substrate is supplied to apost-composting process, resulting in an organic fertiliser that iscomparable to conventional compost.

The batch operation means the individual fermenters must be shut downfrom time to time, that is to say the biogas production must be stopped,the fermented biomass must be removed from the respective fermenter, andthe fermenter must be filled with fresh biomass, with biogas productionbeing resumed. During this process, it is necessary for safety reasonsto prevent an explosive biogas/air mixture from being created while theindividual fermenters are being loaded and unloaded.

For this purpose, it is known from EP 1301583 B for a fermenter that isin use to be flooded with off-gas containing carbon dioxide from thecogeneration system that is being operated with biogas, in the event ofan explosion risk, that is to say if air has entered the fermenter.

SUMMARY OF THE INVENTION

Starting out from a biogas installation according to EP 1301583 B, theobject of the present invention is to make safer the process ofunloading consumed biomass from the fermenter, and loading it with freshbiomass.

This object is achieved by the features of the claims.

The biogas installation according to comprises the necessary componentsto allow a fermenter to be shut down and unloaded safely, and likewiseto be started safely.

The measures result in the biogas production and utilisation beingmaintained for as long as possible even during the shutdown and purgingwith off-gas containing carbon dioxide, that is to say thebiogas/off-gas mixture of the fermenter to be shut down is stillsupplied to the biogas consumer until the quality of the mixture fallsbelow a predetermined level. Only when the methane concentration in thebiogas outlet falls below an upper limit value is the biogas lineleading to the biogas consumer disconnected from the biogas outlet. Thebiogas/off-gas mixture, which now just contains a small amount ofmethane, is fed out via an exhaust chimney. This is done until themethane concentration has fallen to a lower limit value, where thebiogas/off-gas mixture now contains virtually no methane. After this,the fermenter to be shut down is purged with fresh air instead of withoff-gas containing carbon dioxide, and the off-gas/biogas/fresh airmixture is fed out via the exhaust chimney until the carbon-dioxideconcentration in the off-gas/biogas/fresh air mixture has fallen to afirst limit value. Only then is the fermenter opened in order to unloadthe consumed biomass and in order to load the fermenter with freshbiomass again. The previous purging processes with off-gas and fresh airallow the fermenter to be opened, unloaded and loaded, without any riskto the operator.

According to one preferred refinement of the invention, when the methaneconcentration reaches the upper limit value, the biogas/off-gas mixtureis not emitted to the environment via the exhaust chimney but is passedto an off-gas flare where it is burnt off. If required, the off-gasflare can be supplied with additional fuel so that combustion alwaystakes place. The biogas/off-gas mixture is burnt off until the methaneconcentration in the biogas/off-gas mixture falls below a medium limitvalue, which is between the upper and the lower limit value.

According to one preferred refinement of the invention, while thebiofermenter that has been shut down is being unloaded and loadedthrough the open loading and unloading opening, fresh air is sucked inand the gas mixture that is sucked in is passed to the exhaust chimneyvia the purging gas inlet or via the biogas outlet. Alternatively, aspecific fresh-air extraction connection can also be provided in thefermenter.

The advantageous refinement of the invention ensures that fresh air issucked in continuously while the fermenter is being unloaded and loaded.

The method according to the invention for (re)starting the fermenterwhich has been shut down safely prevents an explosive biogas/air mixturebeing formed during starting.

This fermenter which has been started again is connected to the biogasline at a fourth methane concentration limit value, which is the same asthe upper limit value

The off-gas for purging the fermenter to be shut down is provided, forexample, by an internal combustion engine.

According to one preferred embodiment of the invention, the off-gaswhich contains carbon dioxide is provided by a biogas processing deviceconnected downstream from the at least one fermenter.

The other dependent claims relate to advantageous refinements of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, features and advantages of the invention will becomeevident from the following description of exemplary embodiments withreference to the drawings, in which:

FIGS. 1 to 7 show schematic illustrations of various operating statesduring the shut down process and during (re)starting of a fermenter;

FIG. 8 shows a schematic illustration of a second embodiment of theinvention with a fermenter; and

FIGS. 9 to 15 show schematic illustrations of various operating statesof a biogas installation having three fermenters during the shut downprocess and while a fermenter is being (re)started.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 to 7 show a first embodiment of a biogas installation accordingto the present invention with a single fermenter 2. The fermenter 2 hasa cuboid shape and is constructed approximately in the form of aprefabricated garage. The fermenter 2 can be filled with biomass 6, andcan be emptied again by means of a tractor shovel through a loading andunloading opening 4 which extends over one of the end faces of thecuboid fermenter 2. Reference is made to WO 02/06439 with regard todetails of the construction of the fermenter 2.

The fermenter 2 also has a biogas outlet 8, which can be connected viavalves 10 to a biogas line 12, a first biogas/off-gas line 14 and asecond biogas/off-gas line 16. The biogas line 12 leads to acogeneration system 18 as a biogas utilisation device. The firstbiogas/off-gas line 14 leads to a bio off-gas chimney 20. The secondbiogas/off-gas line 16 leads to an off-gas flare 22. Furthermore, thefermenter 2 has a purging gas inlet 24, which can be connected viavalves 10 to an off-gas line 26 or to a fresh air line 28. An off-gasfan 27 is arranged in the off-gas line 26, and can be used to pumpoff-gas into the fermenter 2. A fresh air fan 29 is arranged in thefresh air line 28 in order to suck fresh air in from the environment.Off-gas containing carbon dioxide is passed via the off-gas line 26 aspurging gas into the fermenter 2, and fresh air is passed into thefermenter 2 via the fresh air line 28.

The valves 10 are connected to a control device 30, and are opened orclosed by the control device 30. The control device 30 is also connectedto a first measurement sensor 32, which is arranged in the biogas outlet8 and detects the methane concentration in the respective gas mixture.The control device 30 is also connected to a second measurement sensor34, which is likewise arranged in the biogas outlet 8 and detects thecarbon-dioxide concentration in the respective gas mixture. The controldevice 30 is also connected to a third measurement sensor 36, which isarranged in the biogas outlet 8 and detects the gas volume flow in thebiogas outlet. If required, the extraction of gas from the fermenter 2can be assisted by a fan 38 which is arranged in the biogas outlet.

FIGS. 1 to 7 show various phases of the shut down process and process ofstarting the fermenter 2, with active lines and positions of componentsbeing illustrated by solid lines, while lines and positions ofcomponents which are inactive or are shut down are illustrated by dashedlines.

FIG. 1 shows the first phase of shutting down the fermenter 2, in whichoff-gas containing carbon dioxide is pumped via the off-gas line 26 andthe purging gas inlet 24 into the interior of the fermenter 2. Thebiogas outlet 8 is connected, as before, to the biogas line 12, so thatthe biogas/off-gas mixture is passed on to the cogeneration system 18.

Only when the methane concentration detected by the first measurementsensor 32 in the biogas outlet 8 has fallen below an upper limit valueis the valve 10 in the biogas line 12 closed by the control device 30,and the valve 10 in the second biogas/off-gas line 16 is opened, as isillustrated in FIG. 2. In this second phase of shutting down thefermenter 2, the biogas/off-gas mixture is burnt in the off-gas flare22. If required, this combustion process can be assisted by addingadditional fuel.

When the methane concentration detected by the first measurement sensor32 in the biogas outlet 8 has fallen below a medium limit value, thevalve 10 in the second biogas/off-gas line 16 is closed by the controldevice 30 and the valve 10 in the first biogas/off-gas line 14 isopened, as is illustrated in FIG. 3. In this third phase of shuttingdown the fermenter 2, the biogas/off-gas mixture is emitted to theenvironment via the off-gas chimney 20.

When the methane concentration detected by the first measurement sensor32 in the biogas outlet 8 has fallen below a lower limit value, thevalve 10 in the off-gas line 26 is closed by the control device 30 andthe valve 10 in the fresh air line 28 is opened, as is illustrated inFIG. 4. In this fourth phase of shutting down the fermenter 2, fresh airis pumped into the fermenter 2 via the fresh air line 28 and the purginggas inlet 24. The off-gas/air mixture is emitted further to theenvironment via the biogas outlet 8 and the first biogas/off-gas line 14in the off-gas chimney 20.

When the carbon-dioxide concentration detected by the second measurementsensor 34 in the biogas outlet 8 has fallen below a first limit value,the valve 10 in the fresh air line 28 is closed by the control device30, and the loading and unloading opening 4 is opened, as is illustratedin FIG. 5. At the same time, the fan 38 is used to suck fresh air in viathe open loading and unloading opening, and to emit it to theenvironment via the off-gas chimney 20. This prevents biogas residueswhich the fermented biomass still contains from representing a risk tothe operator during the unloading process.

Once the fermenter 2 has been loaded with fresh biomass again, theloading and unloading opening 4 is closed, the connection between thebiogas outlet 8 and the off-gas chimney 20 is maintained via the firstbiogas/off-gas line 14, and the control device 30 opens the valve 10 inthe off-gas line 26, so that the off-gas which contains carbon dioxideis pumped into the fermenter 2, see FIG. 6. This is continued until thecarbon-dioxide concentration detected by the second measurement sensor34 in the biogas outlet 8 reaches or exceeds a second limit value.

When the carbon-dioxide concentration has reached this second limitvalue, the control device 30 closes the valve 10 in the off-gas line 26and in the first biogas/off-gas line 14, and opens the valve 10 in thebiogas line 12, as is illustrated in FIG. 7. The biogas production phasehas therefore been reached again, and the biogas produced in thefermenter 2 is supplied via the biogas line 12 to the cogenerationsystem 18.

In the embodiment described above, all the measurement sensors 32, 34,36 are arranged in the biogas outlet 8. Alternatively, the second andthird measurement sensors 24, 36 can also be arranged in the first andsecond biogas/off-gas lines 14, 16, respectively. FIG. 8 shows analternative refinement of the invention, which differs from theembodiment shown in FIGS. 1 to 7, in that the first and secondbiogas/off-gas lines 14, 16 are combined to form a common biogas/off-gasline 40, before they open into the biogas outlet 8. The secondmeasurement sensor for detection of the carbon-dioxide concentration isarranged in the common biogas/off-gas line 40, and the third measurementsensor 36 is arranged in the first biogas/off-gas line 14. Apart fromthis, the second embodiment of the invention corresponds to the firstembodiment. The method of operation is also identical.

FIGS. 9 to 15 show a third embodiment of a biogas installation accordingto the present invention, in which three fermenters 2-1, 2-2 and 2-3 areprovided, and are operated in parallel. Mutually correspondingcomponents are provided with the same reference symbols. In the biogasinstallation shown in FIGS. 9 to 15, each of the three fermenters 2-i isprovided with a purging gas inlet 24-1, 24-2 and 24-3, which can each beshut off by a valve 10. The three purging gas inlets 24-i are combinedto form a common purging gas inlet 42. An off-gas line 26 and a freshair line 28 open into the common purging gas inlet 42, and can each beshut off by a valve 10.

Each of the three fermenters 2-i is provided with a biogas outlet 8-1,8-2 and 8-3, which can each be shut off by a valve 10. The firstbiogas/off-gas line 14 to the off-gas chimney 20 and the secondbiogas/off-gas line 16 to the off-gas flare 22 are combined to form acommon biogas/off-gas line 40 in which a fan 38 is arranged. Downstreamfrom the fan 38, the common biogas/off-gas line 40 splits into a first,a second and a third biogas/off-gas line element 40-1, 40-2 and 40-3.The first biogas/off-gas line element 40-1 opens between the valve 10and the first biofermenter 2-1 into the first biogas outlet 8-1. Thesecond biogas/off-gas line element 40-2 opens between the valve 10 andthe second biofermenter 2-2 into the second biogas outlet 8-2. The thirdbiogas/off-gas line element 40-3 opens between the valve 10 and thethird biofermenter 2-3 into the third biogas outlet 8-3. The threebiogas/off-gas line elements 40-1, 40-2 and 40-3 can each be shut off bya valve 10. The three biogas outlets 8-1, 8-2 and 8-3 open into a commonbiogas line 12, which leads to a cogeneration system or combined heatand power unit 18. An exhaust line 44 from the cogeneration system 18opens into a second off-gas chimney 46. The off-gas line 26 is connectedvia a 3-way valve 48 to the exhaust line 44, that is to say the off-gascontaining carbon dioxide which occurs in the cogeneration system 18 isused to purge a fermenter 2-i that is to be shut down. The 3-way valveallows the volume flow of the off-gas which is sent via the off-gas line26 in order to purge a fermenter 2-i, and the amount of off-gas which isemitted via the second off-gas chimney 46 to the environment, to beregulated.

A first measurement sensor 32 is arranged in the common biogas line 12in order to detect the methane concentration. A second measurementsensor 34 for detection of the carbon-dioxide concentration, a thirdmeasurement sensor 36 for detection of the volume flow, and a fourthmeasurement sensor 50 for detection of the methane concentration, arearranged in the common biogas/off-gas line 40, downstream from the fan38 in the flow direction. The four measurement sensors 32, 34, 36 and 50are connected to a control device 30. The various valves 10 are likewiseconnected to the control device. These control lines are not shown inFIGS. 9 to 15, for clarity reasons.

FIGS. 9 to 15 illustrate the processes for shutting down and restartingthe second fermenter 2-2, with FIGS. 9 to 15 showing the same phases andoperating states as FIGS. 1 to 7. The biogas production in the first andthird fermenters 2-1 and 2-3 takes place continuously while the secondfermenter 2-2 is being shut down and started again.

FIG. 9 shows the first phase of shutting down the fermenter 2-2, inwhich off-gas containing carbon dioxide from the cogeneration system 18is pumped via the 3-way valve 48 and the off-gas line 26, the off-gasfan 27 and the second purging gas inlet 24-2 into the interior of thefermenter 2-2. The second biogas outlet 8-2 is connected, as before, tothe common biogas line 12, so that the biogas/off-gas mixture issupplied further to the cogeneration system 18.

Only when the methane concentration detected by the first measurementsensor 32 in the common biogas line 12 has fallen below an upper limitvalue is the valve 10 in the second biogas outlet 8-2 closed by thecontrol device 30, and the valve 10 in the second biogas/off-gas lineelement 40-2 and in the second biogas/off-gas line 16 is opened, as isillustrated in FIG. 10. In this second phase of shutting down thefermenter 2-2, the biogas/off-gas mixture is burnt in the off-gas flare22. If necessary, this combustion process can be assisted by addingadditional fuel.

When the methane concentration detected by the fourth measurement sensor50 in the common biogas/off-gas line 40 falls below a medium limitvalue, the valve 10 in the second biogas/off-gas line 16 is closed bythe control device 30 and the valve 10 in the first biogas/off-gas line14 is opened, as is illustrated in FIG. 11. In this third phase ofshutting down the fermenter 2-2, the biogas/off-gas mixture is emittedto the environment via the off-gas chimney 20.

When the methane concentration detected by the fourth measurement sensor50 in the common biogas/off-gas line 40 has fallen below a lower limitvalue, the valve 10 in the off-gas line 26 is closed by the controldevice 30, the 3-way valve 48 is switched appropriately, and the valve10 in the fresh air line 28 is opened, as is illustrated in FIG. 12. Inthis fourth phase of shutting down the fermenter 2-2, the fresh air fan29 pumps fresh air into the fermenter 2-2 via the fresh air line 28 andthe purging gas inlet 24. The off-gas/air mixture is furthermore emittedto the environment via the second biogas outlet 8-2, the secondbiogas/off-gas line element 40-2, the common biogas/off-gas line 40 andthe first biogas/off-gas line 14, in the off-gas chimney 20. Ifnecessary, this can be assisted by the fan 38.

When the carbon-dioxide concentration detected by the second measurementsensor 34 in the common biogas line 40 has fallen below a first limitvalue, the valve 10 in the fresh air line 28 is closed by the controldevice 30, and the fresh air fan 29 is switched off, as is illustratedin FIG. 13. The loading and unloading opening, which is not illustratedin FIGS. 9 to 15, is opened. At the same time, fresh air is sucked in tothe common biogas/off-gas line 40 via the fan 38 and via the openloading and unloading opening, and is emitted to the environment via theoff-gas chimney 20.

This prevents any biogas residues which the fermented biomass stillcontains representing a risk to the operator during the unloadingprocess. Exhaust gases from a tractor shovel which is used for loadingand unloading are therefore also sucked out.

Once the fermenter 2-2 has been loaded with fresh biomass again, theloading and unloading opening is closed, the connection between thesecond biogas outlet 8-2 and the off-gas chimney 20 via the secondbiogas/off-gas line element 40-2, the common biogas/off-gas line 40 andthe first biogas/off-gas line 14 is maintained, and the control device30 opens the valve 10 in the off-gas line 26, and switches the 3-wayvalve 48 in the exhaust line 44 of the cogeneration system 18, so thatoff-gas containing carbon dioxide is pumped into the fermenter 2-2, seeFIG. 14. This process continues until the carbon-dioxide concentrationdetected by the second measurement sensor 34 in the commonbiogas/off-gas line 40 reaches or exceeds a second limit value.

When this second limit value for the carbon-dioxide concentration hasbeen reached, the control device 30 closes the valve 10 in the off-gasline 26, switches the 3-way valve 38, closes the valve 10 in the secondbiogas/off-gas line element 40-2 and opens the valve 10 in the secondbiogas outlet 8-2, as is illustrated in FIG. 15. This means that thesecond fermenter 2-2 has also once again reached the phase of biogasproduction, and the biogas produced in the fermenter 2-2 is supplied viathe biogas line 12 to the cogeneration system 18. The biogas outlet 8-2is not connected to the common biogas line 12 until the methaneconcentration detected by the fourth measurement sensor 50 has reached afourth limit value. This fourth limit value matches the upper limitvalue.

The valve 10 in the off-gas line 26 may be omitted since its functioncan also be carried out by the 3-way valve 48.

Instead of directly connecting the biogas line 12 to the cogenerationsystem 18 it may be connected first to a gas processing device (notshown) to improve the gas quality. The biogas with improved quality isthen fed to the congeneration system 18. Exhaust gas or off-gas from thegas processing device may be fed to the off-gas line 26.

The following text gives examples of numerical values of the variouslimit values:

-   -   Methane concentration: upper limit value 30% to 50%        -   medium limit value 10% to 20%        -   lower limit value 0% to 3%        -   fourth limit value 30% to 50%    -   Carbon-dioxide concentration: first limit value 0.5% to 2%        -   second limit value 5% to 15%

The off-gas volume flow in the off-gas line 26 is between 150 and 1000m³/h, depending on the size of the fermenter and the amount of off-gasavailable. The fresh air volume flow in the fresh air line 28 is between1000 and 5000 m³/h.

1. Method for shutting down a fermenter in a biogas installation, thebiogas installation comprising the fermenter which operates on theprinciple of dry fermentation for production of biogas in the batch modewith a biogas outlet and a purging gas inlet; a biogas line connected tothe biogas outlet, an off-gas line by means of which off-gas containingcarbon dioxide is supplied to the purging gas inlet, an off-gas chimneyconnected to the biogas outlet via a first biogas/off-gas line, anoff-gas flare connected via a second biogas/off-gas line to the biogasoutlet, a fresh air line connected to the purging gas inlet, a controldevice for connection of the biogas outlet to the biogas line or to anoff-gas chimney via the first biogas/off-gas line or to the off-gasflare via the second biogas/off-gas line and for connection of thepurging gas inlet to the off-gas line or to the fresh air line, ameasurement device which is connected to the control device and has afirst measurement sensor for detection of the methane concentration anda second measurement sensor for detection of the carbon dioxideconcentration in a gas mixture emerging from the at least one fermenter,and a loading and unloading opening, said method comprising the methodsteps of: a) maintaining the connection between the biogas outlet andthe biogas line; b) connecting the off-gas line to the purging gas inletof the fermenter to be shut down; c) purging of the fermenter to be shutdown with off-gas from the off-gas line until the methane concentrationdetected by the first measurement sensor has fallen to an upper limitvalue; d) disconnecting the biogas line from the biogas outlet of thefermenter to be shut down; e) connecting the biogas outlet of thefermenter to be shut down to the first biogas/off-gas line, and supplyof the off-gas/biogas mixture to the off-gas chimney until the methaneconcentration detected by the first measurement sensor has fallen to alower limit value, the lower limit value lower than the upper limitvalue; f) disconnecting the off-gas line from the purging gas inlet ofthe fermenter to be shut down; g) connecting the fresh air line to thepurging gas inlet of the fermenter to be shut down, and supply of freshair into the fermenter to be shut down, until the carbon-dioxideconcentration detected by the second measurement sensor has fallen to afirst limit value; h) opening of the loading and unloading opening ofthe fermenter that has been shut down; and i) unloading the used biomassfrom the fermenter.
 2. Method according to claim 1, characterised inthat the following method steps are carried out between method step d)and method step e): d1) connecting the biogas outlet of the fermenter tobe shut down to the second biogas/off-gas line, and supplying theoff-gas/biogas mixture to the off-gas flare until the methaneconcentration detected by the first measurement sensor has fallen to amedium limit value which is between the upper and the lower limit value;and d2) disconnecting the biogas outlet of the fermenter to be shut downfrom the second biogas/off-gas line.
 3. Method according to claim 1,further comprising the method steps of: j) connecting the fresh air lineto the purging gas inlet and/or to the biogas outlet; and k) supplyingfresh air into the fermenter that has been shut down via the loading andunloading opening by sucking out the gas mixture via the purging gasinlet and/or the biogas outlet while the fermenter that has been shutdown is being unloaded and loaded.
 4. Method according to claim 3,characterised in that, during method step k), the volume flow of theair/biogas/off-gas mixture sucked out via the purging gas inlet and/orthe biogas outlet is monitored by the control device.
 5. Methodaccording to claim 1, characterised in that the off-gas line isconnected to the exhaust pipe of an internal combustion engine. 6.Method according to claim 1, characterised in that the off-gas line isconnected to the exhaust pipe of a biogas processing device whichproduces an off-gas containing carbon dioxide.
 7. Method according toclaim 1, characterised in that the off-gas line is connected to theexhaust of a fuel cell.
 8. Method for starting a fermenter in a biogasinstallation, the biogas installation comprising the fermenter whichoperates on the principle of dry fermentation for production of biogasin the batch mode with a biogas outlet and a purging gas inlet; a biogasline connected to the biogas outlet, an off-gas line by means of whichoff-gas containing carbon dioxide is supplied to the purging gas inlet,an off-gas chimney connected to the biogas outlet via a firstbiogas/off-gas line, an off-gas flare connected via a secondbiogas/off-gas line to the biogas outlet, a fresh air line connected tothe purging gas inlet, a control device for connection of the biogasoutlet to the biogas line or to an off-gas chimney via the firstbiogas/off-gas line or to the off-gas flare via the secondbiogas/off-gas line and for connection of the purging gas inlet to theoff-gas line or to the fresh air line, a measurement device which isconnected to the control device and has a first measurement sensor fordetection of the methane concentration and a second measurement sensorfor detection of the carbon dioxide concentration in a gas mixtureemerging from the at least one fermenter, and a loading and unloadingopening, said method, comprising the steps of: a) loading the fermenterwith fresh biomass b) closing of a loading and unloading opening; c)connecting the biogas outlet to the first biogas/off-gas line; d)connecting the off-gas line to the purging gas inlet of the fermenter tobe started and supply of off-gas to the fermenter to be started untilthe carbon-dioxide concentration detected by the second measurementsensor reaches a second limit value which is higher than the first limitvalue; e) disconnecting the off-gas line from the purging gas inlet; f)disconnecting the first biogas/off-gas line from the biogas outlet; andg) connecting the biogas line to the biogas outlet.
 9. Method accordingto claim 8, characterised in that method step g) is carried out when themethane concentration detected by the first measurement sensor exceeds acogeneration limit value which is equal to or higher than the upperlimit.
 10. Method according to claim 9, characterised in that thecogeneration limit value of the methane concentration is equal to theupper limit value of the methane concentration.